In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist) by the changes of either optical properties or surface physical properties of the resist. Alternatively, the imaging step may use a resistless process such as etched grating or nano-imprint technology. Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc.
The measurement and inspection step after development of the resist (or substrate surface in the case of etching), referred to as in-line because it is carried out in the normal course of processing production substrates, typically serves two purposes. Firstly, it is desirable to detect any target areas where the pattern in the developed resist is faulty. If a sufficient number of target areas are faulty, the substrate can be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a process step, e.g., an etch, with a faulty pattern. Secondly, the measurements may allow errors in the lithographic apparatus, e.g. illumination settings or exposure dose, to be detected and corrected for in subsequent exposures. However, many errors in the lithographic apparatus cannot easily be detected or quantified from the patterns printed in resist. Detection of a fault does not always lead directly to its cause. Thus, a variety of off-line procedures for detecting and measuring errors in the lithographic apparatus are known. These may involve replacing the substrate with a measuring device or carrying out exposures of special test patterns, e.g., at a variety of different machine settings. Such off-line techniques take time, often a considerable amount, during which the end products of the apparatus will be of an unknown quality until the measurement results are made available. Therefore, in-line techniques, ones which can be carried out at the same time as production exposures, for detecting and measuring errors in the lithographic apparatus, are usually preferred.
Scatterometry is one example of an optical metrology technique that can be used for in-line measurements of CD and overlay. There are two main scatterometry techniques:
(1) Spectroscopic scatterometry measures the properties of scattered light at a fixed angle as a function of wavelength, usually using a broadband light source such as xenon, deuterium, or halogen based light source such as a xenon arc lamp. The fixed angle can be normally incident or obliquely incident.
(2) Angle-resolved scatterometry measures the properties of scattered light at a fixed wavelength as a function of angle of incidence, usually using a laser as a single wavelength light source.
The structure giving rise to a reflected spectrum is reconstructed, e.g., using real-time regression or by comparison to a library of patterns derived by simulation. Reconstruction involves minimization of a cost function. Both approaches calculate the scattering of light by periodic structures. The most common technique is Rigorous Coupled-Wave Analysis (RCWA), though light scattering can also be calculated by other techniques such as Finite Difference Time Domain (FDTD) or Integral Equation techniques.
A problem with known angle resolved scatterometry techniques is that they only detect one wavelength at a time so spectra with more than one wavelength have to have those wavelengths time-multiplexed, which increases the total acquisition time taken to detect and process the spectra. In spectroscopic scatterometry, an extended light source with a large etendue is used. Since a small grating must be illuminated with a small spread in angle of incidence, a lot of light from this extended source is wasted. This results in low light levels on the detector that lead to long acquisition times, which have a negative impact on throughput. If short acquisition times are chosen, the measurement results might not be stable.